Natural rubber is typically produced in tropical countries such as Thailand, Malaysia, and Indonesia. Although natural rubber exhibits low viscosity immediately after the production thereof, a phenomenon called storage hardening, whereby the viscosity of the natural rubber increases (gelification), occurs due to storage, shipping, or the like after a period of approximately one month has passed.
In order to solve this problem, Japanese Patent Publication H6-256570A discloses technology which attempts to suppress increases in the viscosity of natural rubber by adding a hydrazide compound to the natural rubber. However, there is still room for improvement in the viscosity-stabilizing effect in this technology. In particular, there is a problem in that the persistence of the viscosity-stabilizing effect over a long period of time is poor. In addition, hydrazide compounds are self-decomposable, and decomposition progresses unless the compounds are prepared as aqueous solutions, which makes it impossible to store the compounds stably. Hydrazide compounds also have a unique odor and give off a foul odor when mixed.
On the other hand, since the raw materials of natural rubber are natural products, there is also a problem in that natural rubber gives off a unique foul odor due to putrefaction or the like during storage or during the production process. This odor diminishes the operating environment in the factory, and the effects on the vicinity of the factory are also problematic. Furthermore, even among manufacturers of rubber products using natural rubber, a foul odor is also generated at the time of processing—in particular, the mastication step—of the acquired natural rubber. Due to urbanization in the vicinities of factories in recent years, there has been immense investment in the installation of deodorization facilities or the like as a countermeasure to this odor, and there has been an earnest demand to make natural rubber odorless.